In the control of a hybrid electric vehicle which includes a traction motor, a traction battery (which may include many individual battery elements), and an engine-driven electrical generator for charging the battery, it has been found that a simple battery charge controller which responds to the battery voltage fails to maintain the batteries at a controlled charge level, in that the batteries slowly discharge during operation, due to the action of control system integrators in the presence of a fluctuating battery or bus voltage. Also, such a simple controller is inefficient from an energy standpoint, in that the battery operating point cannot be controlled, which is equivalent to saying that the battery impedance cannot be controlled.
Improved battery charge control methods are desired for hybrid electric vehicles.